The sulfate-reducing bacteria have a complex electron transfer system which leads to the reduction of either organic substrates or molecular hydrogen. These bacteria can either consume or produce molecular hydrogen. The central part of this electron pathway for Desulfovibrio gigas is constituted by hydrogenase (3 X (4Fe-4S)), cytochrome c3 (4 hemes, with different redox potentials) and a one (4Fe-4S) cluster ferredoxin. This ferredoxin is isolated in different oligomeric forms, which stabilize different oxidation states with different physiological roles, the trimer Fd I being involved in the production of H2 and the tetramer Fd II being more efficient for the consumption of H2. The proposed research aims to understand the mechanism of the structural control of the redox potential for the different hemes of multi-heme cytochromes (c3 and c7). Structural studies of the oligomeric forms of the (4Fe-4S) ferredoxin should allow an understanding of the control of the polypeptide chain over the stabilization of different oxidation states of the cluster. The proteins involved contain intrinsic probes and are particularly suitable for spectroscopic studies. Although a number of tecniques will be used, we shall particularly rely on high resolution NMR. A spectroscopic study of protein-protein hydrogenase should be important to understand the electron pathway through cytochrome c3. This study should also permit an understanding of the kinetics of electron transfer between the hemes and the iron-sulfur centers. Of special interest is a kinetic study of the electron exchange between intramolecular hemes. Any hypothesis developed from this work can be tested for physiological relevance in this active system. An understanding of this electron transfer system clearly has environmental, health, and economic ramifications. Another goal of this research is to help elucidate the mechanism of electron transport in mitochondria, through understanding this simpler system.